For the UK, specifically for Heathrow, we have many different types of separations between pairs of departing aircraft.
- Wake turbulence, which can never be reduced.
- IMC Separations based on direction of turn and speed difference (which are time based, not radar based, so they could be applied if the tower had suffered a radar failure.
- 3nm radar separation for diverging aircraft, which requires radar(!)
- Visual separation (which is I guess what you refer to as VMC separation) which can be applied to reduce everything apart from wake turbulence.
So, a DVR followed by a MID departure from 27R in IMC is 2 minutes. The DVR turns inside the MID route. With radar I can reduce that to 3nm when the second aircraft appears on radar (which would probably be equal to about 60-70 seconds if both mediums), and then in VMC I can apply visual separation to reduce it even further to 'wheels up' which for mediums is about 45 seconds, provided I have another form of separation when I pass the aircraft to their departure sectors.